Neutron source



2,957,096 Patented Oct. 18, 1960 NEUTRON SOURCE Nels K. Bernander and Royce J. Jones, Oak Ridge, Tenn., assignors to the United States of America as represented by the United States Atomic Energy Commission Filed June 10, 1954, Ser. No. 435,958

6 Claims. (Cl. 313-61) This invention relates to neutron sources, and more particularly to a system for producing neutrons through target bombardment with deuterons.

Heretofore in the prior art, it has been the practice in the production of neutrons by target bombardment to employ a collector or target plate with one of its surfaces coated with a thin film of some compound such as lithium deuteride which will give the deuteron-deuteron reaction to produce neutrons. However, this arrangement is primarily suited to high energy, low current bombardment. Although the number of neutrons produced at the target is proportional to current, high current bombardment leads to excessive heating at the target. Increased heat of the lithium deuteride will result in the decomposition of this compound, and escape or loss of the target coating. Cooling of the target would tend to minimize this loss, but coated targets of this character do not readily lend themselves to cooling due to their relatively poor heat conducting characteristics. There are other problems which arise in the use of coated targets. They include the necessity for preparing the plate to receive the coating, the preparation of the coating in suitable form for application to the plate, and finally, the building up of the coating in proper proportions. With the coated target plate, it is necessary to protect the coating from deterioration or contamination such as the effects of oxidation, corrosion, or the accumulation of undesired foreign matter. It may even be necessary to keep the target under vacuum in order to protect it, or in some cases to restore the coating by reprocessing or cleaning after it has accumulated undesirable deposits of film.

Applicants with a knowledge of these problems of the prior art have for an object of their invention the provision of an arrangement for producing neutrons by bombardment of an uncoated target.

Applicants have as another object of their invention the provision of a source of neutrons produced by bombarding a target with deuterons under conditions which will cause them to be embedded therein and facilitate the deuteron-deuteron reaction.

Applicants have as another object of their invention the provision of a system for producing neutrons in substantial quantities by employing larger current flows for bombardment and utilizing a target which may be adequately cooled.

Applicants have as a further object of their invention the provision of a target for a system for producing neutrons which eliminates the necessity for cleaning or storing in order to restore or preserve its characteristics.

Applicants have as a still further object of their invention the provision of a neutron source of the target type wherein deuteron bombardment of the target is of sufficient intensity and energy to embed them in the target to such a depth and density so that for a state of equilibrium an adequate number of them are available for subsequent reaction with later arriving deuterons and increase the production of neutrons.

Applicants have as a still further object of their invention the provision of a target for a neutron source which has a low diffusion rate for deuterons.

Other objects and advantages of our invention will appear from the following specification and accompanying drawings, and the novel features thereof will be particularly pointed out in the annexed claims.

Fig. 1 is a sectional elevation of a conventional type of electromagnetically operated system which has been adapted to the production of neutrons.

Fig. 2 is a schematic of our improved system for producing neutrons.

Fig. 3 is a schematic of a suitable form of ion source for use in our improved system.

Applicants system for the production of neutrons contemplates the provision of a source of deuterons. The deuterons from the source are accelerated against a plate type of target at such speeds as to embed them therein. Subsequent ions upon reaching the target are caused to react with the embedded ions. The deuteron is a particularly effective projectile for causing nuclear transmutations because a relatively small amount of energy is sufiicient to cause its rupture into a neutron and proton. The production of neutrons by deuteron bombardment was first accomplished in connection with Li and Be targets.

The production of neutrons by deuteron bombardment makes available neutron intensities many times greater than the older method of alpha bombardment. The deuteron is an effective particle for irradiation and in many instances is more effective than other particles because the associated neutron has no potential barrier to penetrate yet contributes considerable excitation energy to a compound nucleus. The deuteron-deuteron reaction is particularly advantageous since there is no threshold and at low voltage exhibits a Gamow exponential-type increase. Since the deuteron-deuteron reaction product is two particle the neutrons are monoenergetic. The deuteron-deuteron process has twocompetitive reactions.

The two reactions have approximately the same crosssection thus the yield of neutrons and protons is essentially equal.

Now referring to the schematic of our improved system shown in Fig. 2, the present arrangement comprises an ion source generally designated 1, an accelerating electrode 2, positioned adjacent the exit of the ion source,

and a target 3 located at the focal point of the ion beam from source 1, together with a source of supply 4 of deuterium for the ion source 1.

Any suitable form of hot cathode type of ion source 1 may be employed in our improved system. The one disclosed in the patent granted to Jones et al., No. 2,716,197, is a satisfactory type of source. Another suitable source is shown in Fig. 3. In that arrangement, neutral vapors are fed into an ionizing chamber through a feed line 5. These vapors are bombarded by an electron stream generated therein and electrons are ejected or knocked from the atoms of these vapors, forming ions in the ionization chamber 8. The source of electrons is a heated filament I I which may be supplied with current from any suitable power source through leads generally indicated at 32 in Fig. 1. The filament is positioned behind a slotted partition 9. The electrons emitted from the filament are initially drawn towards the slotted partition 9 by the difference of potential existing between it and the filament, and are accelerated through the slot in the partition, striking an arc across the ionization chamber 8 to the floating plate 13 behind the slotted partition 14 The resulting negative charge built up on the plate 13 tends to reflect the electrons, which have not encountered an ionizing collision, back across the chamber 8, thus increasing the probability of producing deuterons. When the arc is established across the chamber, the diiference in potential between the arc and the filament maintains the flow of electrons. Thus, it is seen, that during the travel of the electrons emitted from the heated filament 11, across the space defined by the chamber 8, that ionization of the neutral vapors occurs. The ions formed in the chamber 8 by the collision of electrons with neutral atoms of gas are then ejected through the exit slit 10 of the source 1 in the usual manner, and are acted upon by the electrostatic fields of the system to accelerate them.

The design and geometry of the accelerating electrode 2 is prescribed by the characteristics of the ion source 1, the conditions of operation, and the overall design of the particular system, as a complete unit. The arrangement of the preferred embodiment of applicants invention includes a water-cooled target 3, at approximately the 180 point, in a position that will exclude all particles except those of the desired charge-to-mass ratio. The source of deuterium 4 is an electrolytic cell. The electrolytic cell 4 contains heavy water, D 0, with a suitable electrolyte. A direct current is caused to flow between the electrodes of the cell by the application of an appropriate potential, causing D to be given otf in the form of a gas at the negative electrode and O to be given oif at the positive electrode of the cell. The gaseous D is then led through line 5 to the source 1 where it is ionized in the manner indicated above. Since the slotted accelerating electrode 2 is maintained at a high negative potential, positively charged ions of deuterium are accelerated from the source 1 through the slit and pass through slot of the accelerating electrode 2 through the magnetic field at right angles thereto. Being acted upon by the field, the ions are focused into a beam and strike the target 3 at the focal point of 180.

The structural embodiment of this system may take the form shown in Fig. l, and may utilize many of the components of a magnetically operated system for the large scale separation of isotopes. In this arrangement 21 designates a conventional tank or enclosure which is maintained at a high vacuum pressure by the usual system of vacuum pumps (not shown). Threading the vessel 21 is a magnetic field which passes into the plane of the paper and is set up by an electromagnet, one pole of which is indicated at 24'. The magnet is of such intensity as to constrain uniform velocity deuteron ions projected within the vessel to an arc whose radii is a predetermined fixed function of the momentum of the ions.

The deuteron ion source is generally indicated at 1 and is mounted on an insulator 26' which projects through a wall of the vacuum vessel 21' and is held in sealed relation with respect thereto by a flanged packing ring 27' which is drawn, by studs 30' threaded into the wall of vessel 21, against an annular packing element 28' to distort it and efiect a seal against leakage into the vessel around the insulator 26'. Where the source 1' is subjected to excessive heating, a pair of water lines generally indicated at 31', may serve to circulate coolant through passages in the Walls of the source. However, since this is conventional in electromagnetically operated equipment for the separation of isotopes, a detailed structural showing has been omitted. Leads 32, rom a power supply to the electrodes including the anode and heating elements of the source, are indicated as passing up through the insulator 26'. They serve to supply heating current for the filament, and to provide a diflerence of potential between cathode and anode to draw electrons, emitted by the filament, to the anode.

The ion accelerating electrode may be of ring-shape, and is indicated at 33'. It is preferably mounted on and carried by a wire or mesh type of cage 34' which also serves to supply a large negative potential to the electrode. As will be seen, the cage 34' serves to enclose unearth",

the deuteron beam 35' and maintain a uniform field free region (from the tank 21' which is maintained at ground potential) throughout the travel of the deuteron until it reaches the target 36'. The target end of the cage 34' may be enclosed by a metallic wall 37 through which tubular target support 38 passes. This support is mounted on an insulator 39', similar to insulator 26, which serves to mount deuteron source 1. The mounting 46, 47 and 48' is also similar to that employed to mount insulator 26. The insulator serves to house tubes 40' which enter the outer free end of the insulator and which circulate coolant from an external source through passages 41 to the chamber 42 of the target 36. It also serves as an electrical conductor for connecting cage 34' and target 36' to lead 43 from the high voltage source 44-. The potential of deuteron source 1 is maintained by lead 45 which passes through insulator 26 and connects to the high voltage source 44'.

The operation of the system has already been described in connection with the schematic arrangement of Fig. 2. It includes the ionization of atoms of deuterons in the ionization chamber 8, the ejection of the resulting ions through the exit slit 10, and the acceleration of the ions into a magnetic field which extends into the paper, so that they are caused to move in an arcuate path within the cage 34, and are then brought to impinge upon the target 36' and embed themselves therein. At the target, the deuteron-deuteron reaction takes place, and neutrons are released or made available.

In some tests and heat transfer studies, it was noted that an uncoated metal target produced very large neutron yields, comparable to or even larger than from a lithium deuteride target. A further check of this effect with other metals gave a thick target yield as follows:

Neutrons/ sec./ ma.

Stainless steel l3.4 10 Copper 8.7x l0 Nickel 6.0 X 10 Tantalum 4.3 X 10 Lithium deuteride 8.1 10

Apparently the explanation for this effect is that deuterons driven into the metal by their bombardment momentum are retained in sufficient quantities to act as target particles. The deuterium may be regarded as trapped at high pressure or as absorbed in the metal. At a constant target temperature, the neutron intensity is proportional to the square of the current, since the amount of target deuterium and the production of neutrons are each proportional to the beam strength. For example, at one microampere the neutron flux would be 10* times that at one milliampere. The flux would thus be only of the order of background in the former case, but more than a million neutrons per second in the latter.

Higher neutron production was achieved by means of a more intense deuteron beam and a higher accelerating voltage. The voltage increase was achieved by operating the ion source 1 at a high (40 kv.) positive potential and the target 3 at a high (20-40 kv.) negative potential. To date the maximum total neutron yield has been 1.2 1O neutrons/second, at 250 ma. beam current and 58 kv. total voltage. Maximum efficiencies in neutron production occurred at relatively low beam currents (approximately 10 ma.) because of the increase in temperature of the target.

Neutrons from the deuteron-deuteron (D,D) reaction are mono-energetic and free of gamma rays and, therefore, uses might include the determination of shielding attenuation data and the measurement of inelastic scattering. It may be noted that the bombardment of trapped deuterons in a plain metallic target tends to eliminate the problems of preventing contamination and oxidation which customarily arise with coated and other types of targets such as lithium deuteride. Further, the use of a thin metallic plate having good heat conduction characteristics, simplifies the problem of cooling. While any suitable metal, such as those listed above, may be employed as the target, it should have the characteristics of permitting beam penetration. The deeper the penetration, the longer the diffusion path for the deuteron, and the greater the probability of its retention by the target until the deuteron-deuteron reaction has taken place. This contributes to a lower dilfusion rate, and lower diifusion rate means that fewer deuterons escape or are lost to the reaction.

Having thus described our invention, we claim:

1. A system for producing neutrons by target bombardment comprising a high intensity source of deuterons, a metal target of deuteron absorbing material located at a point remote from the source, an accelerating electrode for removing the ions from the source and accelerating them towards the target, and means for setting up a magnetic field perpendicular to the direction of travel of the deuterons to thread the space between the source and the target to collimate the ions into a continuous beam and direct them into the body of the target for interaction with each other therein to produce neutrons.

2. A system for producing neutrons by target bombardment comprising a housing, a high intensity source of deuterons positioned in the housing, a coating free metallic member of deuteron absorbing material throughout positioned in spaced relation to the source in the housing to provide a. target, means for accelerating the ions in their travel from the source to the target, and means for collimating the ions into a beam, whereby upon striking the target they embed themselves therein and react with each other to release neutrons.

3. A system for producing neutrons by target bombardment comprising a housing, a high intensity source of deuterons disposed Within the housing, a coating free metallic element of deuteron absorbing material throughout and being of such thickness as to provide a deuteron dilfusion path of sufiicient length to permit interaction of ions therein to form neutrons positioned in spaced relation to the source in the housing to provide a target, means for accelerating the ions in their travel from the source to the target, and means for collimating the ions into a beam.

4. A system for producing neutrons by target bombardment comprising a metallic housing at ground potential having a chamber maintained at a high vacuum pressure, means extending through the walls of the housing for mounting a high intensity deuteron source and a target of a material pervious to the passage of deuterons in spaced relation to the source, said target being of substantially the same material throughout and of sufiicient thickness to provide a deuteron dilfusion path, of sufiicient length to permit interaction of ions therein to form neutrons, an accelerating electrode adjacent the source 5 for accelerating the ions toward the target to provide a continuous beam for impingment thereon, a magnetic field threading the housing to collimate the ions into a beam, and an electrostatic shield extending between the accelerating electrode and the target and maintained at the potential of the accelerating electrode for providing a field free region for the deuteron beam.

5. A method of producing neutrons by target bombardment comprising the steps of feeding deuterium to a limited ionization zone, bombarding the deuterium with a concentrated beam of electrons, accelerating the deuterons from said zone through a magnetic field t-o collimate them into a continuous high intensity beam, and then directing the high intensity ion beam on a deuteron pervious metal target of substantially the same material throughout to embed the deuterons therein and react them to produce neutrons.

6. A method of producing neutrons by target bombardment comprising feeding deuterium from a source to a limited ionization zone, bombarding the deuterium with a concentrated beam of electrons to ionize the gases to form deuterons, accelerating the deuterons from the zone into an electrostatic field free region under the influence of a magnetic field extending normal to their paths of travel to collimate them into an arcuate continuous beam, and then directing the beam against a deuteron pervious target of suflicient thickness and of substantially the same material throughout to provide a diffusion path located at the focal point of the beam, of suflicient length to 35 permit interaction of the ions to produce neutrons.

References Cited in the file of this patent UNITED STATES PATENTS 2,211,668 Penning Aug. 13, 1940 2,240,914 Schutze May 6, 1941 2,251,190 Kallman et .al. July 29, 1941 2,331,189 Hipple Oct. 5, 1943 2,489,436 Salisbury Nov. 29, 1949 2,689,918 Youmans Sept. 21, 1954 2,712,081 Fearon et .al June 28, 1955 OTHER REFERENCES Mobley: Proposed Method for Producing Short Intense Mono-Energetic Ion Pulses, Physical Review, vol.

88, No. 2, Oct. 15, 1952, pages 360-61. 

